(a) Field of the invention:
The present invention relates to an objective to be used for an IC mask testing device with a flying spot scanner employing a fluorescent substance.
(b) Description of the prior art:
An objective for an IC mask testing device should have high resolving power, flat image surface from the center of the field to the marginal portion, small distortion and long working distance due to the fact that the mask patterns are very minute. Besides, chromatic aberration should be corrected satisfactorily and, moreover, chromatic aberration should be corrected favourably even for the wavelength of the fluorescent light to be used.
Microscope objectives disclosed in Japanese published unexamined patent applications Nos. 45741/72 and 45742/72 are known as lens systems having lens configurations resembling the lens configuration of the objective according to the present invention. These known objectives are usable at low magnifications about 20.times. and 40.times.. However, at a high magnification about 140.times. which is an objective of the present invention, said known objectives are not usable because they cannot fulfill the above-mentioned requirements. Furthermore, none of known microscope objectives with high magnifications about 100.times. have a satisfactorily long working distance which is also an object of the present invention. Moreover, there is no known objective of which chromatic aberration for the fluorescent wavelength is corrected favourably in addition to chromatic aberration in general.